Drills for metal working, particularly drills comprising cutting edges forming concave curves in drill axis end view are disclosed in the following Patent Documents 1 and 2 etc.
In a twist drill disclosed in Japanese Patent Laying-Open No. 2-124208 (Patent Document 1), it is assumed that the web thickness of the drill is 20 to 35% of the drill diameter and the shape of a spiral flute in drill axis end view is concaved on a cutting edge outer end with respect to a perpendicular line orthogonal to a straight line connecting the cutting edge outer end and the drill center with each other, the distance from the aforementioned perpendicular line to the wall surface of the spiral flute is set to 45 to 65% of the drill diameter while the flute width ratio is set to 0.9 to 1.2, and the radius of curvature around the bottom portion of the spiral flute coming into contact with a virtual column expressing the web thickness is set to 15 to 20% of the drill diameter, in order to unforcedly curl and treat chips.
In a drill disclosed in Japanese Patent Laying-Open No. 2001-96411 (Patent Document 2), a spiral flute is sigmoidally spread from a portion in contact with the web thickness toward a heel portion in axis-perpendicular sectional view of the drill, a protrusion is provided on the heel portion of the flute, and the distance from the forward end of the protrusion to a drill rotational direction front portion of the spiral flute is defined, in order to prevent chipping of the heel portion resulting from gripping of chips and cancel insufficient lubrication of the spiral flute (chip discharging flute) with a coolant.    Patent Document 1: Japanese Patent Laying-Open No. 2-124208    Patent Document 2: Japanese Patent Laying-Open No. 2001-96411